Kirsten, 22, knew something strange was happening at about eight weeks into her pregnancy. The classic first trimester symptoms, such as weight gain, food aversions and nausea, were all arriving as expected – but she wasn’t the only one affected.

Her partner, Silas, 23, was experiencing similar physical shifts. He started gaining weight and felt repelled by familiar foods. As Kirsten’s morning sickness took hold, he was struck with equally debilitating nausea. “I felt sick every day for weeks,” he says.

Initially, Kirsten was sceptical. She worried that Silas’s symptoms were an elaborate joke, or that he was being insensitive. “But, as time went on, I realised he was truly suffering,” she says. “The nausea was the worst of it. He had it much worse than I did. I ended up sharing my morning sickness medication with him so he could get through the day.”

Silas’s experience was no freak phenomenon. He had Couvade syndrome, otherwise known as a “sympathy pregnancy”. The mysterious – yet surprisingly common – condition causes pregnant women’s partners to manifest the physical and psychological symptoms of pregnancy. Those with the syndrome report abdominal pain, morning sickness, bloating and lethargy, as well as mood swings, memory loss and depression. In more extreme cases, they can even experience pseudocyesis, in which their stomach swells throughout their partner’s pregnancy, only to begin retracting after the birth.

For Kalu, 25, the symptoms were brief, but all-consuming. During the first trimester of his partner’s pregnancy, he struggled with extreme anxiety and nausea. “My stomach was twisting and turning,” he says. “I was throwing up for days. The only thing I could eat were liquids, such as water and fruit smoothies.”

It is easy to dismiss these men as simply being melodramatic, especially when you compare Couvade syndrome with the intense physical strain of a real pregnancy. Perhaps that is why there has been so little research into the causes.

“I don’t think that people understand the nature of the disorder,” says Dr Arthur Brennan, a senior lecturer in nursing at Kingston University, who has written several research papers on the condition. “It sort of straddles the boundaries between a mental disorder and a physical disorder. It doesn’t fit tidily into one or the other category.”

As for how many men may be affected, the research is surprising: one study found that up to 52% of US fathers will experience some symptoms of Couvade syndrome, with 59.1% in Jordan and up to 61% in Thailand. In Poland, a 2013 study found that 72% of expectant fathers could experience at least one pregnancy-related symptom. Lack of research in the UK means that recent statistics are sparse, but estimates in the early 1970s put rates between 11% and 50%. Of course, there is a lot of room for interpretation when symptoms such as mood swings and depression are involved.

The syndrome is not a modern-day one. There are references to Couvade rituals in Corsica, Cyprus, Papua New Guinea and ancient Iberia that stretch as far back as 50BC, with expectant fathers apparently lying in bed throughout the pregnancy, and receiving the same level of attention as mothers. In some cases, they would even wear their partner’s clothes, groan and cry out, and complain of labour pains.

Couvade syndrome may not be limited to heterosexual human couples, although they make up the majority of the reported cases. Dr Bartlomiej Piechowski-Jozwiak, a neurologist at Cleveland Clinic Abu Dhabi, says the most severe case of Couvade syndrome he has ever heard of came via a vet: “A woman developed a variant of Couvade syndrome triggered by her dog’s pregnancy: she had all the symptoms of the syndrome, and she knew she was not pregnant.”

So what does cause Couvade syndrome? For a long time, it was believed to be purely psychological. Some early explanations, rooted in psychoanalytic theory, suggested that the symptoms were born out of a man’s envy of the woman’s ability to procreate. Others suggested it could be a way of diverting attention back to the man, who can feel irrelevant or marginalised during pregnancy.

But more recent studies have suggested that Couvade syndrome could be linked to empathy and attachment. Symptoms, it seems, are more prevalent in men with greater investment in their unborn baby, and increased involvement in the pregnancy.

This was the case for Mike, 32. The tattoo artist had Couvade syndrome with all three of his partner Amanda’s pregnancies, experiencing morning sickness, pseudocyesis, exhaustion and heightened emotions. He believes it may have been down to the couple’s closeness. “We breathe each other’s air constantly,” he says. “We own a couple of businesses together, we live together, we work together and we have children together, so we’re very close. We also spend a lot of our free time together, too, because we’re friends.”

There are potential physical explanations for Couvade syndrome. What we know from the very few studies that have been carried out so far is that men’s hormonal levels shift significantly during the antenatal period. Testosterone tends to diminish, while oestrogen and prolactin rise.

“I’d say my oestrogen levels were all over the place when Amanda was pregnant,” says Mike. “I was an emotional wreck.”

“My training regime practically stopped because I didn’t feel like I had the urge or energy. I’m quite a testosterone-y bloke: I’m mad on the gym, and I train hard and often. But when Amanda is pregnant, that just stops. My ‘want’ just disappears. I just get emotional and lazy, and eat cake.”

Dr Robin Edelstein, a psychologist from the University of Michigan, has studied these hormonal shifts in expectant fathers. The lower testosterone, she suggests, may be to blame for some of the symptoms. “Lower testosterone is associated with weight gain and depression,” she says. “It could [also] make men more supportive and more invested in their relationship, and more prepared to become a parent.”

“A number of studies have found that testosterone is lower,” Brennan agrees, “whereas prolactin tends to rise. The link, or the apparent trigger, is found in men who had the greatest concern or responsiveness to the unborn baby. Possibly, this could be to do with empathy, but it could be due to anxiety as well.”

But for those with Couvade syndrome, it’s not really about finding answers. Instead, the priority is removing the stigma that surrounds the condition, and for it to be acknowledged as a legitimate part of the pregnancy process.

“I don’t really think it’s a cause for research,” says Mike. “Obviously, some people may take the piss, or say I’m trying to steal the thunder from Amanda’s pregnancies. And, yeah, they are her pregnancies, but I can’t deny what’s going on.”

If anything, he says, the syndrome should be celebrated as a symbol of intimacy between two parents (the couple are now working on a book about how to stay close during pregnancy, which they credit to the experience). “Couvade syndrome taught me to be more compassionate; to have more understanding of what it was Amanda was going through – not just physically but mentally,” he says. “Maybe Couvade syndrome will give men more of an understanding of what a woman goes through when she pushes her body to its limits.”